This invention relates, in general, to quantum well structures, and more particularly, to a modulation doped quantum well which is useful in enhanced conductivity structures made from semiconductor materials.
Modulation doping is often used in quantum well structures to provide charge carriers to the quantum well. A modulation doped quantum well has doping atoms formed in a barrier region outside of the quantum well, yet close enough to the quantum well so that the charge carriers can tunnel through the barrier. These charge carries then fall into the quantum well where they alter the conductivity of the quantum well region. Modulation doping is particularly advantageous because doping atoms are not located in the quantum well itself, resulting in improved charge carrier lifetime and mobility in the quantum well.
One problem with modulation doping, however, is that it modifies the energy band structure both inside the well and in the barrier region which surrounds the well. The modified band structure can result in new energy bands developing near the edges of the well which in turn trap charge carriers near the edges. In some quantum well applications it is advantageous to have a high charge carrier density at the center of the quantum well rather than at the edges. Also, the energy band modification caused by modulation doping can result in reduced charge coupling, and therefore poorer superconducting performance. Examples of such enhanced conductivity structures are described in copending U.S. Pat. applications Ser. Nos. 411,780 and 501,59 by the inventor of the present invention and assigned to the same assignee, and incorporated herein by reference.
Enhanced conductivity results in a crystal lattice when electrons, which are normally in energy levels within plus or minus k.sub.B T of a fermi energy (E.sub.F), collapse into an energy state which is approximately k.sub.B T.sub.C below E.sub.F, where T.sub.C is the critical temperature at which superconductivity occurs and k.sub.B is the Boltzmann constant. Electrons can only collapse by coupling or pairing into what is known as "Copper pairs". Cooper pairs require each of the paired electrons in an energy level to have equal and opposite momentum and spin which results in a tighter electron packing density than is possible under normal conditions. Paired electrons scatter with equal and opposite change in momentum. Thus paired electrons carry charge in the energy range k.sub.B T.sub.C with zero resistance.
Accordingly, a key feature of enhanced conductivity materials is an ability to allow electrons to exist as paired electrons rather than fermions, which are responsible for normal resistive charge conduction. Much work has been done recently to develop materials in which paired electrons can exist at high temperatures. The present invention deals with a method of promoting the formation of paired electrons at temperatures where such pairing would not naturally occur.
Electrons in a crystal lattice have a characteristic coherence length which is determined by electronic and crystallographic properties of the crystal lattice. External forces such as heat and electromagnetic fields affect this electron coherence length. Electrons in normal conduction states repel each other, and will not come close enough to form pairs. Electrons in enhanced conductivity materials, however, interact with lattice vibrations (phonons) to form pairs. Paired electrons are separated from each other by the electron coherence length. In naturally occurring superconductors, for example, electronphonon interactions result in superconductivity at low temperature, where the electron coherence length is sufficiently long.
The present invention uses semiconductor materials to provide a synthetic lattice which is adapted to promote electron-phonon interaction, and in particular, to promote electron-phonon interactions which result in formation of paired electrons in materials and at temperatures where paired electrons do not normally exist. More specifically, the present invention comprises a method and structure for increasing charge carrier density near the center of a quantum well, where useful electron-phonon interactions are more likely to occur.
Accordingly, an object of the present invention is to provide a new enhanced conductivity structure having a higher number of electron-phonon interactions.
Another object of the present invention is to provide a modulation doped quantum well having increased charge carrier density near the center of the quantum well.
A further object of the present invention is to provide a modulation doped quantum well having an energy band leveling structure.
Yet another object of the present invention is to provide an enhanced conductivity quantum well with improved tolerance to material composition and wider energy bands.
Yet a further object of the present invention is to provide a resonant superlattice superconductor with quantum wells having phonon generators with an optical longitudinal phonon energy equal to spacing between two energy states.
A still further object of the present invention is to provide an enhanced conductivity material having a more uniform spacing between two energy states through the entire width of the quantum well.